Phase-controlled slot antenna



Aug. 5, 1969 w. F. MCMAHON 3,460,149

PHASE-CONTROLLED SLOT ANTENNA Filed Aug. 12, 1966 FIG. 3

mvmoa William F. McMahon AGENT United States Patent 3,460,149 PHASE-CONTROLLED SLOT ANTENNA William F. McMahon, Silver Spring, Md., assignor to the United States of America as represented by the Secretary of the Navy Filed Aug. 12, 1966, Ser. No. 572,154 Int. Cl. H01q 13/10, 1/42 US. (:1. 345-768 2 Claims ABSTRACT OF THE DISCLOSURE This invention relates to microwave antennas, and more particularly to a phase controlled slot antenna which produces a tilted or off-axis beam pattern.

One of the most critical problems in aero-space communication systems involves the reduction in size of components. Antennas, perhaps more than any other one element, contribute significantly to the bulk of a system. Placement of an antenna in a missile is very critical because of the conflicting requirements of space and direction of beam pattern. In the past it has been the practice to position an antenna behind an R-F transparent nose cone in order to obtain alignment of the directivity pattern with the flight path of the missile. This results in a low-density packaging at least within the nose cone and hence a very uneconomical use of the total volume within the missile. It has been recognized that in aeroballistic missiles exact alignment of the antenna directivity pattern and the flight path is not required. Since the missile flies in an arcuate path, it is only necessary that the directivity pattern of the antenna be in approximate alignment with the slant range to the intended target. By mounting a slot antenna flush in the nose cone of the missile, a certain amount of forward inclination of the antenna beam pattern is obtained naturally due to the gradual tapering of the nose cone. Greater beam tilt in the forward direction is required, however. Heretofore, it has been the practice to obtain the additional needed beam tilt through electrical phasing by means of long line arrays several wavelengths in length. There are several obvious disadvantages to this type of antenna system including size, complexity and cost.

It is therefore an object of the instant invention to pro- Vide a means of obtaining far field radiation patterns from an element length of only approximately one wavelength in length with substantial beam tilt away from broadside.

It is another object of this invention to provide a small, flush-mounted microwave antenna which produces a tilted or off-axis pattern that may be advantageously employed in an aeroballistic missile by providing a certain degree of forward look-ahead.

It is a further object of the invention to provide an antenna characterized by its small size and simplicity which produces radiation patterns previously obtainable only with longer arrays.

According to the present invention, the foregoing and other objects are attained by providing a cavity-backed, standing-wave slot radiator approximately one wavelength in length having an ofl-center feed. A variable ICC tuning reactance is positioned within the cavity and further enhances the tilt of the pattern and permits adjustment of the VSWR. By adjusting the variable reactance and the off-center feed, phase and amplitude of the aperture excitation may be readily varied in a manner that will produce the desired width and angle in the radiation pattern main beam.

The specific nature of the invention, as well as other objects, aspects, uses and advantages thereof, will clearly appear from the following description and the accompanying drawing, in which:

FIG. 1 is a plan view looking into the cavity through the slot of an antenna according to the invention;

FIG. 2 is a cross-sectional View taken along section line 2-2 in FIG. 1;

FIG. 3 is a polar plot of a typical radiation pattern obtained with the antenna shown in FIG. 1; and

FIG. 4 illustrates the appplication of an antenna according to the invention in an aeroballistic missile.

Referring now to the drawing wherein like reference numerals designate identical or corresponding parts throughout the several views, and more particularly to FIG. 1, the antenna 10 has a cavity resonator body 11 which is conveniently constructed in the form of an open faced rectangular box having side-walls and backwall. The open face constitutes the radiating slot 12. The body 11 may be made of aluminum, brass or other suitable material and may be milled from solid stock, drawn or otherwise formed in a conventional manner. To facilitate mounting, tabs 13 and 14 are provided at either end of the antenna body 11. When used in an aerospace vehicle, it is preferrable to mount the antenna 10 flush behind an R-F transparent window in a well-known manner to maintain the continuity of the vehicle skin.

Positioned between opposing longer dimensioned sidewall faces of the antenna body 11 is an R-F feed 15. Feed 15 is mounted off-center relative to both the dimensions of length and depth of cavity body 11 and, as shown in FIG. 2 comprises a coaxial connector 16 screwed through the top face side-wall of antenna body 11. The center conductor of the connector passes into the cavity and extends into a recess in one end of a short, metal rod 17. Rod 17 is supported at the other end by a block of dielectric material 18 which is fixedly attached to the interior surface of the bottom side-wall of antenna body 11. The permit tuning to match the impedance of the antenna 10 with the coaxial feed, a tuning screw 19 is provided. Tuning screw 19 screws into a threaded axial bore extending part way up rod 17. Access to screw 19 is obtained through aperture 20 in the bottom side-wall of antenna body 11 and a central bore through dielectric block 18 which are in axial registry with the bore in rod 17.

A variable tuning reactance 21 is positioned within the antenna cavity in line with R-F feed 15 and spaced apart therefrom along the length of the antenna 10. Tuning reactance 21 may, as illustrated in FIG. 1, simply comprise a machine screw 22 threaded through the top longer dimensioned side-wall of antenna body 11 and locked in place by a lock nut 23.

The principal feature of this invention which is the tilted or off-axis radiation pattern of the antenna is accomplished by the ofi-center feed of the cavity-backed slot. The dimensions indicated in FIGS. 1 and 2 are, therefore, more or less critical. These are tabulated below referenced to the operating wavelength:

A=0.93 c=0.127 A1=0.62 X=0.073 B=0.20 0:031

The above dimensions have been found experimentally to be optimum but may be varied within reasonable limits to obtain similar satisfactory results. The variable tuning reactance 21 may be omitted since it is theoff-center feed that is primarily responsible for the tilted beam pattern. The reactance 21, however, is desirable because it further enhances the tilt of the pattern and additionally permits adjustment of the VSWR.

A typical radiation pattern 24 of an antenna constructed according to the invention is shown in the polar plot in FIG. 3. The axis of the pattern is shown as displaced from broadside by an angle 0. Measured values for the angle 0 have been between 18 and 31 depending on the presence and adjustment of the variable reactance 21. Side lobe radiation has been observed to increase with increasing angle 0.

FIG. 4 illustrates an antenna 10 mounted flush in the nose cone of a missile 25. The radiation pattern 24, tilted in the forward direction, provides substantial forward lookahead. Note that the tapering nose cone of missile 25 adds to the angle 0.

It will be apparent that the embodiment shown is only exemplary and that various modifications can be made in construction and arrangement within the scope of the invention as defined in the appended claims.

I claim as my invention:

1. A slot antenna having an operating wavelength x comprising an open-faced rectangular shaped cavity of dimensions A, B, and C+X wherein A is substantially equal to .93)\, B is substantially equal to .20A and C is substantially equal to .127 wherein the open face is defined by the A and B dimension edges and said open face defines an energy radiating slot, and

means for feeding energy into said cavity extending through a wall thereof of dimensions A and C-i-X, said means being located on said wall at a distance A/ 3 from one extremity of said wall. of dimension C-l-X measured along the A dimension direction, and at a distance X from said open face, wherein X is substantially equal to measured along the C+X dimension direction. 2. A slot antenna according to claim 1 further comprising means for tuning said cavity extending through said wall, said means being located at a distance /3 A from said extremity of said wall of dimension C-l-X measured along the A dimension direction.

References Cited UNITED STATES PATENTS 2,573,460 10/1951 Lindenblad 343-768 XR 2,629,051 2/1953 Lindenblad 343767 2,972,147 2/ 1961 Wilkinson 343-767 3,239,838 3/1966 Kelleher 343-789 3,262,119 7/1966 Sisson 343768 ELI LIEBERMAN, Primary Examiner M. NUSSBAUM, Assistant Examiner US. Cl. X.R. 343-789 

